The present invention relates to blends comprising a polar derivative of an ethylene based polymer and cycloolefin polymers, and to their applications in sealants for packaging films.
In the packaging industry, it is conventional that a product is packaged in a plastic film or a rigid plastic package which is then heat sealed. The film forming the package usually comprises a substrate layer, a tie-layer and a sealant layer. The sealant layer is the topmost layer of the film and is in direct contact with the packaged item. It must have a high perforation resistance, especially for packaging of dry food. The seal must also possess a mechanical resistance sufficient to maintain the tight-seal properties during storage and transport. It is preferable that the package be opened without destroying the integrity of the film itself. In other words, the seal should be peelable, that is to say that it should be easily openable by pulling with hands, without the need of scissors or other instruments.
In addition, in industry, it is desired that these packaging films be suitable for high speed machine packaging. In this view, it is useful to provide a sealant layer that can be sealed at relatively low temperatures and that has a very high hot tack in order to allow for higher packaging line speeds of packaging machines. It would therefore be useful to have sealant layers made out of polymers showing a relatively low heat seal initiation temperature.
It is known to mix cyclic olefinic copolymers and classical polyolefins in order to manufacture sealant layers for packaging films. Nevertheless, the heat seal initiation temperature of those sealant layers are still very high. They are usually around 100xc2x0 C. Moreover, the perforation resistance of these films is also not satisfactory.
Therefore, there is still a need for sealant layers for packaging films that would have a relatively low heat seal initiation temperature, a good perforation resistance and also a good hot tack value.
Now, it has been surprisingly found that the combination of two incompatible specific types of polymers lead to a sealant layer showing excellent properties as regards stiffness, perforation resistance, heat seal strength and hot tack strength.
Incompatible polymers, as used herein, means polymers that are not soluble in each other, in other words, polymers having solubility parameters which are significantly different from each other, preferably which differ from at least 0.5 units. Solubility parameters are defined in J Bandrup, E. H. Immergut (Ed), Polymer Handbook, Page IV 337 ff New York, 1975.
A first aspect of the invention is a blend comprising at least a polar derivative of an ethylene based polymer and at least a cycloolefin polymer.
Another aspect of the invention is a film comprising the blend above.
Another aspect of the invention is a sealant layer comprising the above film.
Another aspect of the invention is a packaging comprising the sealant layer above.
Another aspect of the invention is a method for improving the perforation resistance of a film made of a polar derivative of an ethylene based polymer comprising the step of introducing therein from about 0.1 to 50% by weight, relative of the weight of the film, of a cycloolefin polymer.
Another aspect of the invention is a method for reducing the heat seal initiation temperature of a film made of a polar derivative of an ethylene based polymer comprising the step of introducing therein from about 0.1 to 50% by weight, relative of the weight of the film, of a cycloolefin polymer.
Another aspect of the invention is a method for stabilizing the heat seal strength of a film made of a polar derivative of an ethylene based polymer over a wide temperature range comprising the step of introducing therein from about 0.1 to 50% by weight, relative of the weight of the film, of a cycloolefin polymer.
Another aspect of the invention is a method for increasing the hot tack strength of a film made of a polar derivative of an ethylene based polymer range comprising the step of introducing therein from about 0.1 to 50% by weight, relative of the weight of the film, of a cycloolefin polymer.
The film of the invention has improved resistance to impact, improved hot tack value. It can also have a significantly reduced heat seal initiation temperature (see Examples).
Although the ethylene based polymer and the cycloolefin polymer of the invention are incompatible, their combination leads to improved mechanical properties of the film made of their blend. Moreover, when opening the seal, the film made of the blends of the invention have the potential to delaminate and therefore offer a perfectly controlled peel use as well as high perforation resistance, which is an advantage over commercial peel systems which are usually poor in perforation resistance.
The film of the invention shows an excellent perforation resistance. The packagings made of this film are therefore more resistant. This is very important in food packaging, where bones for example may damage the film. It also shows a low seal initiation temperature, which means that packages made of this film can be sealed at lower temperatures than before. This is particularly important in industrial processes where each operation must be rapid. The film of the invention also shows high hot tack strength. That means that the sealing operation, in particular in vertical form fill (VFF) machines and in flow wrapping processes where the goods are packaged under slight overpressure in modified atmosphere (MAP), can be speeded up significantly. One does not have to wait a certain time until the sealing is effective. This also allows more packages to be heat sealed in less time. The film can be wrapped in high speed automatic wrapping machines, increasing the productivity in the industrial processes.
The films of the invention can find use in food packagings like cereals, meat, soup packagings.